Scanning tunneling microscopes, scanning atomic force microscopes, and similar other microscopes are collectively known as scanning probe microscopes (SPMs). It is known that a scanning probe microscope is used to permit one to observe the topography of a sample surface as a two-dimensional image (i.e., to generate a topographic image) or to image various kinds of physical forces, such as an atomic force or a magnetic force exerted between a sample and a probe tip.
Various methods are known to derive topographic images of sample surfaces. One of them is known as the slope detection method. One example of instrumentation for effecting this method is shown in FIG. 3. Shown in this figure are a cantilever 1, a probe tip 2, a sample 3, piezoelectric scanning elements 4x, 4y, 4z, a laser 5 acting as a light source, a reflecting mirror 6, a detector 7, an IV (current-to-voltage) amplifier 8, a lock-in amplifier (LIA) 9, an error amplifier 10, driving power supplies 11, 12, a scanning driver portion 13, a control portion 14, a display portion 15, a piezoelectric device 16, and an oscillator 17.
The probe tip 2 is formed at the front end of the cantilever 1. The probe tip 2 and the sample 3 are positioned opposite to each other. The laser 5 is located above the cantilever 1. Laser light emitted from the laser 5 is reflected by the top surface of the cantilever 1, is reflected by the reflecting mirror 6, and impinges on the detector 7.
For example, the detector 7 comprises a photodetector split into four parts A-D, as shown in FIG. 4. A change in the position hit by the laser light can be detected from the state of balance between the amounts of light impinging on the two pairs of parts A-D of the detector. If the probe tip 2 is brought close to the sample 3, a physical force (such as an atomic force or a magnetic force) produced between the atoms at the probe tip 2 and the atoms on the surface of the sample 3 acts to deflect the cantilever 1. It is well known that a change in the position hit by the reflected laser light induced by the vertical deflection of the cantilever 1 can be sensed by comparing the amount of light impinging on one detector part pair (A+B) with the amount of light impinging on the other detector part pair (C+D).
Assume that the detector 7 includes a calculational means for performing given calculations on the output signals from the two pairs of parts of the detector. In this case, this detector 7 performs the following calculation: EQU ((A+B)-(C+D))/((A+B)+(C+D)) (1)
The piezoelectric device 16 is attached to a member that supports the cantilever 1. This piezoelectric device 16 is vibrated by a signal generated from the oscillator 17. That is, the cantilever 1 is forced to vibrate by the piezoelectric device 16. The oscillation frequency of the oscillator 17 is set close to the resonance frequency of the cantilever 1.
Referring still to FIG. 3, the output current from the detector 7 is converted into a voltage by the IV amplifier 8 and phase detected by the lock-in amplifier 9 by reference to the output signal from the oscillator 17. The output signal from the lock-in amplifier 9 is fed to one input terminal of the error amplifier 10. A preset reference voltage V.sub.REF is applied to the other input terminal of the error amplifier 10. This error amplifier 10 produces the difference between these two input signals.
The output signal from the error amplifier 10 is supplied to the control portion 14 and to the driving power supply 11. The error amplifier 10 controls the piezoelectric scanning element 4z for z motion via the driving power supply 11 so that the output signal from the lock-in amplifier 9 equals the reference voltage V.sub.REF. Therefore, the distance between the cantilever 1 and the surface of the sample 3 is always held to a distance corresponding to the reference voltage V.sub.REF. That is, the piezoelectric scanning element 4z for z motion is controlled by the signal fed back to it so that the distance between the probe tip 2 and the sample 3 is kept constant.
The control portion 14 creates a topographic image of the sample 3 from the output signal from the error amplifier 10 and displays the image on the display portion 15. Also, the control portion 14 controls the scanning driver portion 13 to scan the sample 3 in two dimensions within the x-y plane. In particular, the scanning driver portion 13 produces a signal for two-dimensional scanning to the driving power supply 12 under control of the control portion 14. In response to this signal, the power supply 12 supplies signals to piezoelectric elements 4x and 4y for x and y motions, respectively.
In this way, a topographic image of the surface of the sample 3 is derived. Where the sample 3 is made of a magnetic material, it is desired to know the magnetic distribution on the sample 3, in addition to having the topographic image. Accordingly, a method for creating a visible two-dimensional image from the magnetic distribution on the sample has been developed.
An example of this is given now. Where the magnetic force should be detected, the probe tip needs to be made of a magnetic material. For this purpose, it is common practice to fabricate the probe tip itself from a magnetic material or to coat a magnetic material on the probe tip.
Where the probe tip is simply fabricated from a magnetic material as described above, the probe tip undergoes an attractive force irrespective of the magnetic polarization characteristics of the surface of the sample. Specifically, the probe tip is attracted, whether the sample surface is south pole or north pole, and therefore the probe tip receives an attractive force. Of course, the variations in the magnetic force can be converted into an image of plural gray levels by detecting the motion of the cantilever responsive to the attractive force. However, it is impossible to know the polarity.
Accordingly, the cantilever, its support member, and the probe tip are all fabricated from a magnetic material as described above. A conductive wire is wound around the support member 18 for the cantilever 1 to form a coil 19, as shown in FIGS. 5(a) and 5(b). An electrical current is fed to the coil 19 from a power supply (not shown in FIGS. 5(a) and 5(b)), thus magnetizing the cantilever 1 and the probe tip 2. Note that FIG. 5(a) is a plan view, while FIG. 5(b) is a side elevation.
Where the probe tip 2 magnetized in this manner is used, variations in the magnetic polarization characteristics of the surface of the sample produce an attractive or repulsive force acting on the probe tip 2, thus moving the cantilever 1 up and down. By detecting this motion of the cantilever 1, an image of various gray levels can be created from the magnetic polarization characteristic distribution of the surface of the sample. This image will be hereinafter referred to as a magnetic image.
Where a topographic image is derived with the structure shown in FIG. 3, the piezoelectric scanning element 4z for z motion is controlled according to the signal fed back to it from the error amplifier 10 to maintain constant the distance between the probe tip 2 and the sample 3. Where a magnetic image is produced, it is necessary to detect only the vertical motion of the cantilever 1 without providing the feedback control of the piezoelectric scanning element 4z. This means that where a topographic image and a magnetic image should be obtained from the same sample, two separate measurements must be performed.